The present invention relates to a process for preparing tactic polymers employing a chiral catalyst. The process is especially suited for preparing multiblock copolymers employing multiple catalysts and a compound known as a “chain shuttling agent” (CSA). Tactic polymers, especially isotactic olefin polymers including multi-block copolymers containing tactic polymer segments, and polymeric blends comprising the same are usefully employed in the preparation of solid articles such as moldings, films, sheets, and foamed objects by molding, extruding, or other processes, and are useful as components or ingredients in laminates, polymeric blends, and other end uses. Multi-block polymers comprise two or more differing regions or segments (blocks) differing in physical or chemical properties, such as crystallinity, tacticity, chain branching, and/or monomer insertion errors, thereby causing the polymer to possess unique physical properties. The resulting products are used in the manufacture of components for automobiles, such as profiles, bumpers and trim parts; packaging materials; electric cable insulation, adhesives, and other applications.
It is known in the art to prepare multi-block copolymers from one or more monomers by the use of chain shuttling agents under suitable polymerization techniques, especially continuous solution polymerization conditions at relatively high monomer conversions. Preferred processes employ two catalysts having differing comonomer incorporation properties and a chain shuttling agent such as organo aluminum or organo zinc compounds. Examples include WO2005/904025, WO2005/904026, and WO2005/904027, as well as Science, 312, 714-719 (2006).
In general, for the preparation of tactic polymers, including segments of multiblock copolymers, from C3 and higher α-olefins, preferably propylene, one or more metal compounds containing an asymmetrically substituted tetravalent atom, especially carbon or silicon, are employed. Due to the presence of the asymmetrically substituted atomic center, two enantiomeric molecular forms, minor images of one another and identified as R- and S-enantiomers, exist. The tactic polymers prepared using an equal molar quantity of each enantiomer, referred to as a racemic mixture, differ in spatial orientation but possess identical bulk properties. However, multiblock copolymers made using enantiomeric mixtures comprise polymer segments formed from both enantiomers. This alternating monomer insertion orientation in the same polymer results in inversions in the polymer chain, which ultimately reduces the overall polymer tacticity. The higher the number of such inversions the greater is the loss of tactic properties of the polymers. The highest quantity of inversions will occur through the use of a racemic mixture of enantiomers. The presence of such inversions can interfere with crystal formation and decrease polymer properties such as heat resistance (as measured for example by melting point, Tm), crystallinity and modulus. Moreover, block length or molecular weight is often inferior as well. Finally, functionalization of tactic polymers involving chain termination using a functionalized chain terminating agent or other process can result in two enantiomeric forms of the functionalized polymer, one of which may be preferred for certain end use applications requiring chirality.
It would be desirable if there were provided an improved process for preparing tactic polymers, including multi-block copolymers comprising tactic polymer segments, especially linear multi-block copolymers of one or more C3-10 α-olefins and/or mixtures of one or more C3-10 α-olefins with ethylene and/or a conjugated or non-conjugated C4-20 diene, by the use of a shuttling agent under chain shuttling polymerization conditions, characterized by an improvement in one or more of the foregoing properties. It would also be desirable to provide such an improved process that is capable of preparing multi-block copolymers, especially linear multi-block copolymers of C3-20 α-olefins, having increased molecular weight and longer segment lengths. Moreover, the preparation of tactic polymers using a single enantiomer of a metal complex in a high yield process is desirable as well. Finally, it would be desirable to provide an improved process for preparing any of the foregoing desirable polymer products in a highly productive, continuous solution polymerization process.